I agree with everything Comradehoser has to say on this subject.

I currently have a soft foam Protec Classic. Its taken many hits (I ride street BMX), and held up quite well.
I'm quite sure it has stopped me from recieving multiple concussions.

I also have a Giro bike helmet which weighs about 1/4 as much as the Protec, and offers MUCH more ventilation, but I only wear it when I'm on the road bike and less likely to fall and bonk my head.